Forest ammonia emission

Natural sources of ammonia emissions to the atmosphere are volcanic eruptions, forest fires, and the microbial fixation of nitrogen (Galbally 1985 Hegg et al. 1987 National Science Foundation 1999). Excreta from household pets, wild animals, and humans are also contributing sources (Asman and Drukker 1988 Buijsman et al. 1987 Crutzen 1983). 

Rice can also be an indirect source of N2O (and NO) emissions via deposition of volatilized ammonia on natural ecosystems, particularly wet tropical forests, which are one of the main natural sources of N2O (Table 8.4a). 

Goethel (1980) Various soils, pH 4-7, covered with weeds, in orchards, pastures, and forests 0-7 Measurements mainly during midday acidic soils gave no emissions, but sometimes were sinks snow cover also absorbed ammonia... 

It is likely that natural ecosystems (forest, grassland) emit no or only small amounts of ammonia because normally there is a deficit of fixed nitrogen in landscapes. Reported emissions factors over forests span three orders of magnitude and are likely be influenced by re-emission of wet deposited ammonium. Older publications considerably overestimated emission by using simple models considering soil ammonium concentrations obtained from relative decomposition and nitrification rates, where Henry s law gives the equilibrium concentration of ammonia gas in the soil, and a simplified diffusion equation yields the flux to the atmosphere, for example, Dawson (1977) calculated it to be about 47 Tg N yr b... 

Therefore, (dry) deposition exceeds emission, for example, in forests the soil may emit NH3 but it is up taken by the canopy, and hence remains within the ecosystem. This has been shown in the 1980s, when the European interest in ammonia motivated the first measurements of ammonia dry deposition to semi-natural ecosystems (e. g., Horvath 1983, Duyzer et al. 1987, Erisman and Wyers 1993, Sutton et al. 1993). While the earlier compensation point studies had identified plant stomata as the key exchange site, these European measurements noted large rates of dry deposition that could only be explained by the fact that most of the NH3 was deposited directly onto leaf surfaces. 

---